Commonly, video display terminals (VDTs) are used as input/output terminals for computer and communications systems. Such VDTs usually serve to display alphanumeric information. One form of VDT, the raster scan VDT, requires, as inputs, a video signal containing the information to be displayed, along with horizontal and vertical synchronization signals, driven by a clock signal, which is also used by the VDT. The video signal is an analog signal which causes variations in display intensity along each horizontal scan line.
In the case of an alphanumeric raster scan VDT, these scan-line variations, when displayed parallel to each other, convey the intended alphanumeric information. The video signal is generated in coordination with a character generator, which actually produces a series of analog signals, each corresponding to a different row of dots within a string of characters to be displayed. There are typically seven or nine rows of dots within each character string. An alphanumeric raster-scan VDT can be caused to display images with much higher resolution by allowing each row of dots to be independently created. A VDT with these capabilities can be used to display variable-intensity pictures.
Some VDTs are designed specifically to display pictorial information. Typically these VDTs have very high resolution, e.g., at least one thousand scan-lines, each row containing at least one thousand columns. Terminals having such high resolution capabilities (over 1,000,000 pixels displayable) are costly. To illustrate, the information to be displayed on a high resolution VDT is generally stored in a digital memory device. Raster-scanning such a display requires that the memory elements be read out sequentially and corresponding video signals be produced. Because of the high data rates such an approach requires, the enabling hardware are correspondingly high priced.
Many applications require only a moderate degree of resolution. Low cost VDTs, if provided with the proper signals, can serve this purpose.